The present invention relates to digital information transmission systems including twisted wire pair transmission lines, and in particular to equalization for digital repeaters in the presence of near-end crosstalk.
It is well known that a transmission system involving transmission over symmetric (balanced) wire pairs requires repeaters to be placed at intervals, typically less than 2 km (kilometers) apart. Such repeaters reduce signal distortion by equalization, and by employing amplification restore the signal to a suitable level for retransmission well above the level of the near-end crosstalk from other systems using the same route. FIG. 1 of the accompanying drawing shows a section of such a route in which a repeater 1 transmits high amplitude signals into a symmetric wire pair 3 at point 2. These signals are attenuated while travelling down the wire pair 3 to point 4, where they are received by a subsequent repeater 5. A repeater 6 transmitting at point 7 into another symmetric wire pair 8, in the opposing direction and towards point 9 and a repeater 10, will induce crosstalk into the aforementioned signals near point 4. This crosstalk is near-end crosstalk coupling due to imperfect balance of the cable pairs and is indicated in FIG. 1 by a curved arrow between points 7 and 4. A similar coupling occurs between points 2 and 9. Other crosstalk mechanisms may also exist. For example, near-end crosstalk can occur between systems transmitting in the same direction if transmitted and received signals come within close proximity to each other.
For a digital system the functions of each repeater are usually to re-amplify, regenerate and retime the symbols being transmitted. Conventionally, the function of re-amplification involves equalization (inversion) of the linear filtering effect of the cable pair such that the subsequent pulse spectrum has a predetermined shape meeting Nyquist's criterion for no intersymbol interference. The intersymbol interference is negated by selectively amplifying the higher frequencies by the magnitude of attenuation suffered by them in transmission. As the attenuation of the cable pair increases with frequency, the gain of the re-amplification function correspondingly increases with frequency, at least up to a frequency of approximately one half of the symbol rate. This increasing of gain with frequency is unfortunate, since it simultaneously amplifies crosstalk, while the coupling mechanism producing the crosstalk deteriorates with increasing frequency.